A fault current is generally defined as a temporary and substantial surge in the current transmitted along a power transmission/distribution network. The fault current may be caused by any number of events, including a lightning strike, downed power lines, or a catastrophic failure of one or more components in the network, which results in localized grounding. When such events occur, a large load appears. The network, in response, may deliver a large amount of current or the fault current to this load. This fault current may exceed the capacity of some of the components in the network and destroy the components. One way to minimize the effect of the fault current is to incorporate a fault current limiter (FCL), which may limit the transmission of the fault current. Ideally, the fault current limiter is fast acting, responding within a few milliseconds of the fault condition. In addition, the current limiter should be self-resetting, allowing normal current to be transmitted after the fault condition subsides.
Examples of FCL may include circuit breakers or fuses. During a fault condition, the circuit breaker mechanically opens the network and disrupts further fault current transmission. This system, although effective, may not be fast acting or self-resetting. In particular, there are significant limits to how fast a circuit breaker can open. In the presence of an inductive load, an arc will develop between the contacts and continue to carry current even after the components are not in contact. Also, the circuit breaker must be closed after the fault condition subsides. If fuses are used, the fuses may have to be replaced manually.
Another example of the conventional FCL is an inductive fault current limiter (IFCL) 100 shown in FIG. 1. The conventional IFCL 100 may comprise first and second steel cores 102a and 102b, an AC circuit 104, and a superconducting circuit 106. As shown in the figure, The AC circuit 104 is wound around the outer limbs of the first and second cores 102a and 102b. Moreover, the superconducting circuit 106 is wound around the inner limb of each core 102a and 102b. Generally, the first and second cores 102a and 102b may be made out of steel or other saturable magnetic materials.
In operation, AC current is transmitted through AC circuit 104. At the same time, DC current flows through the superconducting circuit 106 that is wound around the inner limb of the first and second cores 102a and 102b. During normal conditions, DC current flowing through the superconducting circuit 106 maintains the cores 102a and 102b at magnetic saturation, and minimum inductance will be exhibited by the AC circuit 104. During fault conditions, the fault current flowing through the AC circuit 104 take the cores 102a and 102b out of magnetic saturation. As a result, the AC circuit may exhibit large inductance opposing further increase of the AC current flowing through the AC circuit. Through this process, the transmission of the fault current flowing through the AC circuit 104, and the entire IFCL 100, may be reduced.
Another example of the conventional FCL is a superconducting fault current limiter (SCFCL). Generally, the SCFCL contains a superconducting circuit which is maintained below critical temperature level Tc, critical magnetic field level Bc, and critical current level Ic. In addition, the SCFCL includes a shunt that is in parallel with the superconducting circuit. During normal operation, the SCFCL exhibits almost zero resistivity, and the current from the network is directed to the superconducting circuit and transmitted through the SCFCL with almost zero resistivity. During a fault condition, at least one of the temperature, magnetic field, and current is raised above the critical level. In response, the superconducting circuit is quenched, and the resistance of the circuit and the SCFCL surges. As a result, the fault current is directed to the shunt. As the shunt introduces resistance, a current with much lower amplitude exits the SCFCL. The SCFCL is desirable as the system is fast-acting and self-resetting after the fault condition.
Among others, the reliability is an important requirement of any fault current limiting systems. Any defects in the systems may prevent transmission of normal current during normal conditions, making the system highly inefficient. The defects may also prevent the systems from effectively limiting the transmission of a fault current. Accordingly, a new technique for improving reliability of fault current limiting system is needed.